This invention relates to a structure for minimizing separating pressure in a scroll compressor by tapping an intermediate suction pressure to the tip of at least one of the scroll members.
Scroll compressors are becoming widely accepted in the HVAC and refrigeration industries. Scroll compressors are relatively inexpensive, and typically more efficient and less noisy than reciprocating compressor counterparts. Scroll compressor technology has advanced greatly over the past several years. However, scroll compressor design still presents challenges in achieving reliable operation over a broad range of suction and discharge conditions. One major challenge is the reduction of the separating force between the orbiting and fixed scroll members.
FIG. 1 is a view of a known scroll compressor 20. An orbiting scroll 22 is driven through a shaft 24 to move relative to a fixed scroll 26 and compress a fluid captured between orbiting scroll 22 and fixed scroll 26. Fixed scroll 26 has a scroll wrap 28 and the orbiting scroll has a scroll wrap 27. As known, the two scroll wraps contact each other at several points along the flanks, as well as opposing baseplates, that defines compression chambers between fixed and orbiting scroll wraps.
Refrigerant captured between orbiting scroll 22 and fixed scroll 26 creates a separating force tending to move the two scroll members away from each other. It is desirable to maintain the two scroll members in contact with each other to minimize leakage and avoid instability. When a scroll compressor becomes instable, the orbiting scroll is not in equilibrium. Instead, it may pivot or overturn until it comes in contact with another mechanical element. This action, coupled with the orbital movement of the orbiting scroll results in a sort of wobbling motion with axial contact occurring along the edge of the part. This wobbling, or instability, results in leakage through the gaps opened by the separated tips, edge loading of the scroll surfaces, and angular misalignment of the scroll drive bearing. All of these could quickly lead to loss of performance and premature failure of the compressor.
In effect, the separating force tries to push orbiting scroll 22 away from the fixed scroll 26. To combat this separating force, a back pressure chamber 29 is created between two sealing elements 30 and 32 mounted in a crankcase 33 which is also fixed to the fixed scroll 26. Back pressure chamber 29 receives fluid from a tap, such as tap 34. The aspects of compressor 20 described to this point are as known in the art and form no portion of this invention.
However, the back chamber force is limited in magnitude, because of space limitations on the back chamber area and maximum achievable back chamber pressure. Essentially, the force in back pressure chamber 29 must overcome the separating force and press orbiting scroll 22 upwardly against fixed scroll, as well as be high enough to avoid orbiting scroll instability. The problem becomes most pronounced for refrigeration applications, with a broad range of operating pressures. Thus, it would be most desirable to reduce the separating force to minimize the restrictions on the compressor operating range.
The separating force across a portion of the scroll is shown graphically in FIG. 2. The dotted line 28 shows the location over the tip portion of scroll wrap. As is known, a higher pressure is applied on one side of wrap 28 and a lower pressure exists on the opposed side. The separating force is created by the pressure multiplied by the area over which the pressure is applied.
The present invention is directed to reducing the component of separating force applied across the scroll wrap tip. There is a pressure transition or gradient 35 across the tip of scroll wrap 28. The transition can be estimated by assuming a straight slope between the high pressure to the low pressure across the width of the wrap. While this estimation may not always be accurate, it is generally a good approximation. In practice, however, there are some variations and the pressure gradient is not always a constant slope. The problem to be solved by this invention will be explained by reference to the constant pressure slope shown in FIG. 2. However, it should be understood that the slope may be a curve or other irregular shape. The problem to be solved would still exist.
As shown in FIG. 2, there is changing pressure shown by cross-hatching beneath gradient 35 across the width of scroll wrap 28. This pressure multiplied by the area it covers contributes a portion of the separating force. Originally, scroll wraps were thin and of constant width. The separating force component across the scroll wrap tips was relatively small in this type of prior art systems, since the area of the scroll wrap was relatively small.
However, more recently, varying width scroll wraps have been developed such as shown in FIG. 1. These varying width scroll wraps have some relatively wide locations. At the wide locations, the separating force component over the scroll wrap tips becomes significant, and as such it becomes beneficial to reduce it for the reasons mentioned above.